Peter, can you confirm if this is actually applicable to Windows/Linux? As I understand it, immersive mode is an Ash-only feature, and we plan to remove the lightbars corresponding to the tabs (see issue 607029).

Not sure.  There's code in the codebase for immersive-related stuff on Mac, it looks like, and I didn't think the tabstrip code bits were Ash-gated.  Scott, do you know the story on immersive mode?

Clicking the "fullscreen" icon in the Chrome menu does different things on different platforms:

Win/Linux: current tab becomes fullscreen (user must press F11 to exit fullscreen).

ChromeOS: immersive mode is entered. The tabs shrink to narrow horizontal bars, and hovering the mouse near the top of the screen makes the top chrome slide into view.

OSX: the Chrome browser window appears maximized (tabstrip is still fully visible) and the system menu (File, Edit, etc) becomes hidden. Moving the mouse to the top of the screen makes the system menu appear.

Given that this is ChromeOS-specific, and the plan of record is to stop showing the immersive mode version of the tabstrip (narrow bars), I'm merging this into issue 607029.